1. Field of the Invention
The present invention relates to a method of manufacturing a catalyst in which a catalytic metal is supported by a base material, the method being able to improve dispersibility of the catalytic metal in the base material and further able to restrain the catalytic metal from being oxidized during the manufacturing process of the catalyst.
2. Description of the Prior Art
In general, because exhaust gas discharged from automotive engines or the like contains atmospheric pollutants such as HC (hydrocarbon), CO (carbon monoxide), NOx (nitrogen oxides) and so on, regulations on the exhaust gas have been strengthened worldwide in recent years. For example, in the State of California, USA where air pollution causes serious social problems, the corporate average NMOG regulations have been introduced, and further the regulation level of the exhaust gas is going to be strengthened stepwise from now on. In order to cope with these regulations, low-pollution vehicles such as LEV or ULEV must be introduced into the market (to be introduced stepwise from year 1997-year 2000).
Consequently, to an exhaust system of the automotive engine or the like, in general, an exhaust gas purifying device (catalytic converter) using an exhaust gas purifying catalyst is installed to purify the exhaust gas. Hereupon, as the above-mentioned exhaust gas purifying catalyst, conventionally, a catalyst, in which a catalytic metal such as palladium, platinum, rhodium or the like is supported by a porous base material composed of ceria, alumina, zeolite or the like, has been popularly adopted.
Thus, in the specification of Japanese Patent Application No. Hei 9-57485, the applicant of the present application formerly disclosed an exhaust gas purifying catalyst including a lower catalytic layer in which palladium is supported as a catalytic metal by a base material composed of alumina, ceria etc. and an upper catalytic layer in which platinum and rhodium are supported as catalytic metals by a base material composed of ceria, the lower catalytic layer being disposed on a honeycomb-formed carrier, and the upper catalytic layer being disposed on the lower catalytic layer. In the exhaust gas purifying catalyst, due to the above-mentioned construction, its heat resistance is improved, and further its catalytic activity at lower exhaust gas temperature is improved, while improving its NOx purifying performance at higher exhaust gas temperature. In the exhaust gas purifying catalyst described above, palladium is principally used for improving catalytic activity of the catalyst at lower exhaust gas temperature.
However, in the exhaust gas purifying catalyst disclosed in the specification of Japanese Patent Application No. Hei 9-57485 according to the applicant of the present application, there is still such a problem that NOx purifying ratio at higher exhaust gas temperature is not sufficiently improved. That is, for example, as to the exhaust gas purifying catalyst after an aging treatment at high temperature of 1100.degree. C., although its NOx purifying ratio at exhaust gas temperature of 500.degree. C. (namely, C500 purifying ratio) reaches approximately 80% (rig test, that is simulated test), the NOx purifying ratio described above would be required to be a little more improved.
According to knowledge or consideration of the inventors of the present application, a reason why the NOx purifying ratio of the exhaust gas purifying catalyst at higher exhaust gas temperature is not sufficiently improved as described above, may be supposed such that the catalytic metal is not sufficiently atomized because dispersibility of the catalytic metal in the base material is relatively lower. Further, another reason may be supposed such that the catalytic metal is partially oxidized during the manufacturing process of the exhaust gas purifying catalyst (furnacing step etc.) so that catalytic activity of the catalyst is lowered. In consequence, it may be supposed that NOx purifying ratio of the exhaust gas purifying catalyst at higher exhaust gas temperature would be sufficiently improved and further purifying ratio of each of other atmospheric pollutants would be also sufficiently improved, if dispersibility of the catalytic metal in the base material is sufficiently raised and further oxidation of the catalytic metal is restrained or prevented during the manufacturing process of the exhaust gas purifying catalyst.